The proposed experiments will provide new information concerning sarcomere mechanics and the influence of interfilamentary forces in striated muscle contraction. We will use recently developed techniques which allow direct control of the intracellular ionic environment and the sarcomere and interfilamentary spacing (fiber width) of cardiac and skeletal muscle cells. We propose to examine systematically the separate and interactive relationship between force, sarcomere velocity, sarcomere length, interfilamentary spacing (fiber width) and calcium in frog semitendinosus fibers and rabbit ventricular tissues. A statistical thermodynamic approach will be used to model muscle forces. The experiments should provide new insight into the primary factors involved in modulating and regulating sarcomere force and velocity in heart and skeletal muscle.